Electrode assembly for a redox flow battery

ABSTRACT

Methods and systems are provided for a redox flow battery system. In one example, the redox flow battery system includes a cell stack compressed between terminal structures defining ends of the redox flow battery. The cell stack may be formed of a plurality of cells where each cell includes a deformable positive electrode in contact with a first face of a membrane separator and a negative electrode configured to be less compressible than the positive electrode and arranged at a second face of the membrane separator.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Non-Provisionalapplication Ser. No. 17/308,913, entitled “ELECTRODE ASSEMBLY FOR AREDOX FLOW BATTERY,” and filed on May 5, 2021. Application Ser. No.17/308,913 claims priority to Provisional Application No. 63/025,227,entitled “ELECTRODE ASSEMBLY FOR A REDOX FLOW BATTERY,” and filed on May15, 2020. The entirety of the above-listed application is herebyincorporated by reference for all purposes.

FIELD

The present description relates generally to methods and systems for aredox flow battery.

BACKGROUND AND SUMMARY

Redox flow batteries are suitable for grid scale storage applicationsdue to their capability for scaling power and capacity independently, aswell as for charging and discharging over thousands of cycles withreduced performance losses in comparison to conventional batterytechnologies. An all-iron hybrid redox flow battery is particularlyattractive due to incorporation of low cost, earth-abundant materials.The iron redox flow battery (IFB) relies on iron, salt, and water forelectrolyte, thus comprising simple, earth abundant, and inexpensivematerials and eliminates incorporation of harsh chemicals therebyallowing the IFB to impose minimal negative impact on the environment.

An electrochemical cell of the IFB may include a positive electrode, amembrane separator, a negative electrode, and a bipolar plate, arrangedin a stack. In some examples, the bipolar plate may be a net-shapemolded structure, adapted with interdigitated flow field (IDFF) channelsto direct electrolyte flow through the cell. The bipolar plate ispositioned between the negative electrode and a first face of thepositive electrode and the membrane separator is positioned between asecond, opposite face of the positive electrode and a negative electrodeof an adjacent cell. The positive electrode may be formed from a gasdiffusion layer (GDL) paper, for example, which may be compressed ontothe bipolar plate, over the IDFF channels. By implementing the positiveelectrode as a flow-through porous electrode in combination with theIDFF channels of the bipolar plate, a performance of the IFB may beenhanced.

However, the inventors herein have recognized that forming the positiveelectrode from the GDL paper may hinder electrolyte flow through theIDFF channels of the bipolar plate. For example, high GDL papercompression pressures may be needed to enable sufficient electrolytemass transport over the IDFF channels. Such high pressures may demanduse of higher load bearing IFB components, driving up costs. IFB systemcosts may be further exacerbated by the use of costly GDL papermaterials to form the positive electrode.

In one example, the issues described above may be addressed by a redoxflow battery system including a cell stack compressed between terminalstructures defining ends of the redox flow battery, the cell stackformed of a plurality of cells, and wherein each cell of the pluralityof cells includes, a deformable positive electrode having a permeabilitybased on a mechanical compression of the positive electrode and inface-sharing contact with a first face of a membrane separator, anegative electrode configured to be less compressible than the positiveelectrode and arranged at a second face of the membrane separatoropposite from the positive electrode, and an electrolyte in contact withthe positive electrode. In this way, an iron redox flow battery (IFB)system may include a low cost electrode assembly that promotes highbattery performance.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an example redox flow battery systemincluding a battery cell with electrodes and a membrane separator.

FIG. 2 shows a perspective view of a first embodiment of a redox flowbattery system.

FIG. 3 shows a perspective view of a second embodiment of a redox flowbattery system.

FIG. 4 shows an exploded view of the first embodiment of the redox flowbattery system of FIG. 2 .

FIG. 5 shows a rear view of the first embodiment of the redox flowbattery system of FIG. 2 .

FIG. 6 shows a side view of the first embodiment of the redox flowbattery system of FIG. 2 .

FIG. 7 shows an exploded view of a first example of an electrodeassembly which may be implemented in a redox flow battery system.

FIG. 8 shows a partially exploded view of the electrode assembly of FIG.7 with a positive electrode.

FIG. 9 shows the electrode assembly of FIG. 8 fully assembled.

FIG. 10 shows a first cross-section of a portion of a first example of acell stack.

FIG. 11 shows an example of a method for manufacturing an electrodeassembly for a redox flow battery system.

FIGS. 2-10 are shown approximately to scale, however, other dimensionsmay be used as desired.

DETAILED DESCRIPTION

The following description relates to systems and methods formanufacturing a redox flow battery with reduced cost of storage. Theredox flow battery is shown in FIG. 1 with an integrated multi-chambertank having separate positive and negative electrolyte chambers. In someexamples, the redox flow battery may be an all-iron flow battery (IFB)utilizing iron redox chemistry at both a negative and a positiveelectrode of the IFB. The electrolyte chambers may be coupled to one ormore battery cells, each cell comprising the negative electrode and thepositive electrode. The one or more battery cells may be stacked along acommon axis to form a cell stack which may be sandwiched betweenterminal structures such as pressure plates, as shown in FIG. 2 in aperspective view of a first embodiment of the IFB as well as in anexploded view depicted in FIG. 4 . The IFB may have more than one cellstack, as illustrated in FIG. 3 in a second embodiment of the IFB.Electrolyte may flow in and out of the IFB via a plurality of inlets andoutlets in at least one of the pressure plates. The plurality of inletsand outlets are shown in a rear view of the IFB in FIG. 5 and anelectrolyte flow path through the IFB is indicated in FIG. 6 in a sideview of the IFB. Each cell of the cell stack of the IFB may include anelectrode assembly formed of the positive and negative electrodes, abipolar plate, and a membrane separator. The positive electrode may beformed from a sheet of felt, allowing the positive electrode to beporous and deformable with a permeability which may be varied based oncompression. An exploded view of a first example of the electrodeassembly, without the positive electrode, is shown in FIG. 7 . Thepositive electrode may be coupled to the electrode assembly, in contactwith bipolar plates of the electrode assembly, as shown in a partiallyexploded view in FIG. 8 and fully assembled in FIG. 9 . A detailedcutaway view of the cell stack is illustrated in FIG. 10 , showinginterfaces between the components of the electrode assembly and regionsof high and low permeability along the positive electrode. A method formanufacturing a low cost electrode assembly, such as the electrodeassembly of FIG. 10 where the electrode assembly includes a positiveelectrode formed from felt, is shown in FIG. 11 .

FIGS. 2-10 show example configurations with relative positioning of thevarious components. If shown directly contacting each other, or directlycoupled, then such elements may be referred to as directly contacting ordirectly coupled, respectively, at least in one example. Similarly,elements shown contiguous or adjacent to one another may be contiguousor adjacent to each other, respectively, at least in one example. As anexample, components laying in face-sharing contact with each other maybe referred to as in face-sharing contact. As another example, elementspositioned apart from each other with only a space there-between and noother components may be referred to as such, in at least one example. Asyet another example, elements shown above/below one another, at oppositesides to one another, or to the left/right of one another may bereferred to as such, relative to one another. Further, as shown in thefigures, a topmost element or point of element may be referred to as a“top” of the component and a bottommost element or point of the elementmay be referred to as a “bottom” of the component, in at least oneexample. As used herein, top/bottom, upper/lower, above/below, may berelative to a vertical axis of the figures and used to describepositioning of elements of the figures relative to one another. As such,elements shown above other elements are positioned vertically above theother elements, in one example. As yet another example, shapes of theelements depicted within the figures may be referred to as having thoseshapes (e.g., such as being circular, straight, planar, curved, rounded,chamfered, angled, or the like). Further, elements shown intersectingone another may be referred to as intersecting elements or intersectingone another, in at least one example. Further still, an element shownwithin another element or shown outside of another element may bereferred as such, in one example.

Hybrid redox flow batteries are redox flow batteries that arecharacterized by the deposition of one or more of the electro-activematerials as a solid layer on an electrode. Hybrid redox flow batteriesmay, for instance, include a chemical that plates via an electrochemicalreaction as a solid on a substrate throughout the battery chargeprocess. During battery discharge, the plated species may ionize via anelectrochemical reaction, becoming soluble in the electrolyte. In hybridbattery systems, the charge capacity (e.g., a maximum amount of energystored) of the redox battery may be limited by the amount of metalplated during battery charge and may depend accordingly on theefficiency of the plating system as well as the available volume andsurface area available for plating.

As shown in FIG. 1 , in a redox flow battery system 10, a negativeelectrode 26 may be referred to as a plating electrode and a positiveelectrode 28 may be referred to as a redox electrode. A negativeelectrolyte within a plating side (e.g., a negative electrodecompartment 20) of a battery cell 18 may be referred to as a platingelectrolyte, and a positive electrolyte on a redox side (e.g. a positiveelectrode compartment 22) of the battery cell 18 may be referred to as aredox electrolyte. In some examples, the negative electrode 26 may beformed of a mesh and act as a spacer between a first bipolar plate 36and a membrane separator 24. The positive electrode 28 may be formed ofa porous and deformable felt and a permeability of the felt may variedbased on an amount of compression applied to the felt, where thepermeability affects electrolyte flow through the positive electrodecompartment 22 of the battery cell 18. Details of the negative electrode26 and the positive electrode 28 are described further below withreference to FIGS. 7-11 .

Anode refers to the electrode where electro-active material loseselectrons and cathode refers to the electrode where electro-activematerial gains electrons. During battery charge, the positiveelectrolyte gains electrons at the negative electrode 26; therefore thenegative electrode 26 is the cathode of the electrochemical reaction.During discharge, the positive electrolyte loses electrons; thereforethe negative electrode 26 is the anode of the reaction. Alternatively,during discharge, the negative electrolyte and negative electrode may berespectively referred to as an anolyte and anode of the electrochemicalreaction, while the positive electrolyte and the positive electrode maybe respectively referred to as a catholyte and cathode of theelectrochemical reaction. During charge, the negative electrolyte andnegative electrode may be respectively referred to as the catholyte andcathode of the electrochemical reaction, while the positive electrolyteand the positive electrode may be respectively referred to as theanolyte and anode of the electrochemical reaction. For simplicity, theterms positive and negative are used herein to refer to the electrodes,electrolytes, and electrode compartments in redox battery flow systems.

One example of a hybrid redox flow battery is an all iron redox flowbattery (IFB), in which the electrolyte comprises iron ions in the formof iron salts (e.g., FeCl₂, FeCl₃, and the like), wherein the negativeelectrode comprises metal iron. For example, at the negative electrode26, ferrous ion, Fe²⁺, receives two electrons and plates as iron metalon to the negative electrode 26 during battery charge, and iron metal,Fe⁰, loses two electrons and re-dissolves as Fe²⁺ during batterydischarge. At the positive electrode, Fe²⁺ loses an electron to formferric ion, Fe³⁺, during charge, and during discharge Fe³⁺ gains anelectron to form Fe²⁺. The electrochemical reaction is summarized inequations (1) and (2), wherein the forward reactions (left to right)indicate electrochemical reactions during battery charge, while thereverse reactions (right to left) indicate electrochemical reactionsduring battery discharge:

Fe²⁺+2e−↔Fe ⁰−0.44 V (Negative Electrode)  (1)

Fe²⁺↔2Fe³⁺+2e−+0.77 V (Positive Electrode)  (2)

As discussed above, the negative electrolyte used in the IFB may providea sufficient amount of Fe²⁺ so that, during charge, Fe²⁺ can accept twoelectrons from the negative electrode to form Fe⁰ and plate onto asubstrate. During discharge, the plated Fe⁰ may then lose two electrons,ionizing into Fe²⁺ and may be dissolved back into the electrolyte. Theequilibrium potential of the above reaction is −0.44 V and thus, thisreaction provides a negative terminal for the desired system. On thepositive side of the IFB, the electrolyte may provide Fe²⁺ during chargewhich loses electron and oxidizes to Fe³⁺. During discharge, Fe³⁺provided by the electrolyte becomes Fe²⁺ by absorbing an electronprovided by the electrode. The equilibrium potential of this reaction is+0.77 V, creating a positive terminal for the desired system.

The IFB provides the ability to charge and recharge its electrolytes incontrast to other battery types utilizing non-regenerating electrolytes.Charge is achieved by applying a current across the electrodes viaterminals 40 and 42. The negative electrode 26 may be coupled viaterminal 40 to the negative side of a voltage source so that electronsmay be delivered to the negative electrolyte via the positive electrode(e.g., as Fe²⁺ is oxidized to Fe³⁺ in the positive electrolyte in thepositive electrode compartment 22). The electrons provided to thenegative electrode 26 (e.g., plating electrode) can reduce the Fe²⁺ inthe negative electrolyte to form Fe⁰ at the plating substrate, causingit to plate onto the negative electrode 26.

Discharge can be sustained while Fe⁰ remains available to the negativeelectrolyte for oxidation and while Fe³⁺ remains available in thepositive electrolyte for reduction. As an example, Fe³⁺ availability canbe maintained by increasing the concentration or the volume of thepositive electrolyte to the positive electrode compartment 22 side ofcell 18 to provide additional Fe³⁺ ions via an external source, such asan external positive electrolyte tank 52. More commonly, availability ofFe⁰ during discharge may be an issue in IFB systems, wherein the Fe⁰available for discharge may be proportional to the surface area andvolume of the negative electrode substrate as well as the platingefficiency. Charge capacity may be dependent on the availability of Fe²⁺in the negative electrode compartment 20. As an example, Fe²⁺availability can be maintained by providing additional Fe²⁺ ions via anexternal source, such as an external negative electrolyte chamber 50 toincrease the concentration or the volume of the negative electrolyte tothe negative electrode compartment 20 side of cell 18.

In an IFB, the positive electrolyte comprises ferrous ion, ferric ion,ferric complexes, or any combination thereof, while the negativeelectrolyte comprises ferrous ion or ferrous complexes, depending on thestate of charge of the IFB system. As previously mentioned, utilizationof iron ions in both the negative electrolyte and the positiveelectrolyte allows for utilization of the same electrolytic species onboth sides of the battery cell, which can reduce electrolytecross-contamination and can increase the efficiency of the IFB system,resulting in less electrolyte replacement as compared to other redoxflow battery systems.

Efficiency losses in an IFB may result from electrolyte crossoverthrough the separator 24 (e.g., ion-exchange membrane barrier,micro-porous membrane, and the like). For example, ferric ions in thepositive electrolyte may be driven toward the negative electrolyte by aferric ion concentration gradient and an electrophoretic force acrossthe separator. Subsequently, ferric ions penetrating the membranebarrier and crossing over to the negative electrode compartment 20 mayresult in coulombic efficiency losses. Ferric ions crossing over fromthe low pH redox side (e.g., more acidic positive electrode compartment22) to high pH plating side (e.g., less acidic negative electrodecompartment 20) may result in precipitation of Fe(OH)₃. Precipitation ofFe(OH)₃ may degrade the separator 24 and cause permanent batteryperformance and efficiency losses. For example, Fe(OH)₃ precipitate maychemically foul the organic functional group of an ion-exchange membraneor physically clog the small micro-pores of an ion-exchange membrane. Ineither case, due to the Fe(OH)₃ precipitate, membrane ohmic resistancemay rise over time and battery performance may degrade. Precipitate maybe removed by washing the battery with acid, but the constantmaintenance and downtime may be disadvantageous for commercial batteryapplications. Furthermore, washing may be dependent on regularpreparation of electrolyte, contributing to additional processing costsand complexity. Alternatively, adding specific organic acids to thepositive electrolyte and the negative electrolyte in response toelectrolyte pH changes may mitigate precipitate formation during batterycharge and discharge cycling without driving up overall costs.Additionally, implementing a membrane barrier that inhibits ferric ioncross-over may also mitigate fouling.

Additional coulombic efficiency losses may be caused by reduction of H⁺(e.g., protons) and subsequent formation of H₂ (e.g., hydrogen gas), andthe reaction of protons in the negative electrode compartment 20 withelectrons supplied at the plated iron metal electrode to form hydrogengas.

The IFB electrolyte (e.g., FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and the like)is readily available and can be produced at low costs. The IFBelectrolyte offers higher reclamation value because the same electrolytecan be used for the negative electrolyte and the positive electrolyte,consequently reducing cross contamination issues as compared to othersystems. Furthermore, owing to its electron configuration, iron maysolidify into a generally uniform solid structure during plating thereofon the negative electrode substrate. For zinc and other metals commonlyused in hybrid redox batteries, solid dendritic structures may formduring plating. The stable electrode morphology of the IFB system mayincrease the efficiency of the battery in comparison to other redox flowbatteries. Further still, iron redox flow batteries reduce the use oftoxic raw materials and can operate at a relatively neutral pH ascompared to other redox flow battery electrolytes. Accordingly, IFBsystems reduce environmental hazards as compared with all other currentadvanced redox flow battery systems in production.

Continuing with FIG. 1 , a schematic illustration of the redox flowbattery system 10 is shown. The redox flow battery system 10 may includethe redox flow battery cell 18 fluidly connected to a multi-chamberedelectrolyte storage tank 110. The redox flow battery cell 18 maygenerally include the negative electrode compartment 20, separator 24,and positive electrode compartment 22. The separator 24 may include anelectrically insulating ionic conducting barrier which prevents bulkmixing of the positive electrolyte and the negative electrolyte whileallowing conductance of specific ions therethrough. For example, theseparator 24 may include an ion-exchange membrane and/or a microporousmembrane.

The negative electrode compartment 20 may include the negative electrode26, and the negative electrolyte may be formed of electroactivematerials. The positive electrode compartment 22 may include thepositive electrode 28, and the positive electrolyte may be formed ofelectroactive materials. In some examples, multiple redox flow batterycells 18 may be combined in series or parallel to generate a highervoltage or current in a redox flow battery system. Further illustratedin FIG. 1 are negative and positive electrolyte pumps 30 and 32, bothused to pump electrolyte solution through the flow battery system 10.Electrolytes are stored in one or more tanks external to the cell, andare pumped via negative and positive electrolyte pumps 30 and 32 throughthe negative electrode compartment 20 side and the positive electrodecompartment 22 side of the battery, respectively.

The redox flow battery system 10 may also include the first bipolarplate 36 and a second bipolar plate 38, each positioned along arear-facing side, e.g., opposite of a side facing the separator 24, ofthe negative electrode 26 and the positive electrode 28, respectively.The first bipolar plate 36 may be in contact with the negative electrode26 and the second bipolar plate 38 may be in contact with the positiveelectrode 28. In other examples, however, the bipolar plates may bearranged proximate but spaced away from the electrodes within therespective electrode compartments. The IFB electrolytes may betransported to reaction sites at the negative and positive electrodes 26and 28 by the first and second bipolar plates 36 and 38, resulting fromconductive properties of a material of the bipolar plates 36, 38.Electrolyte flow may also be assisted by the negative and positiveelectrolyte pumps 30 and 32, facilitating forced convection through theredox flow battery cell 18. Reacted electrochemical species may also bedirected away from the reaction sites by the combination of forcedconvection and the presence of the first and second bipolar plates 36and 38.

As illustrated in FIG. 1 , the redox flow battery cell 18 may furtherinclude negative battery terminal 40, and positive battery terminal 42.When a charge current is applied to the battery terminals 40 and 42, thepositive electrolyte is oxidized (lose one or more electrons) at thepositive electrode 28, and the negative electrolyte is reduced (gain oneor more electrons) at the negative electrode 26. During batterydischarge, reverse redox reactions occur on the electrodes. In otherwords, the positive electrolyte is reduced (gain one or more electrons)at the positive electrode 28, and the negative electrolyte is oxidized(lose one or more electrons) at the negative electrode 26. Theelectrical potential difference across the battery is maintained by theelectrochemical redox reactions in the positive electrode compartment 22and the negative electrode compartment 20, and may induce a currentthrough a conductor while the reactions are sustained. The amount ofenergy stored by a redox battery is limited by the amount ofelectro-active material available in electrolytes for discharge,depending on the total volume of electrolytes and the solubility of theelectro-active materials.

The flow battery system 10 may further include the integratedmulti-chambered electrolyte storage tank 110. The multi-chamberedstorage tank 110 may be divided by a bulkhead 98. The bulkhead 98 maycreate multiple chambers within the storage tank so that both thepositive and negative electrolyte may be included within a single tank.The negative electrolyte chamber 50 holds negative electrolyte formed ofelectroactive materials, and the positive electrolyte chamber 52 holdspositive electrolyte formed of electroactive materials. The bulkhead 98may be positioned within the multi-chambered storage tank 110 to yield adesired volume ratio between the negative electrolyte chamber 50 and thepositive electrolyte chamber 52. In one example, the bulkhead 98 may bepositioned to set the volume ratio of the negative and positiveelectrolyte chambers according to the stoichiometric ratio between thenegative and positive redox reactions. The figure further illustratesthe fill height 112 of storage tank 110, which may indicate the liquidlevel in each tank compartment. The figure also shows gas head space 90located above the fill height 112 of negative electrolyte chamber 50,and gas head space 92 located above the fill height 112 of positiveelectrolyte chamber 52. The gas head space 92 may be utilized to storehydrogen gas generated through operation of the redox flow battery(e.g., due to proton reduction and corrosion side reactions) andconveyed to the multi-chambered storage tank 110 with returningelectrolyte from the redox flow battery cell 18. The hydrogen gas may beseparated spontaneously at the gas-liquid interface (e.g., fill height112) within the multi-chambered storage tank 110, thereby precludinghaving additional gas-liquid separators as part of the redox flowbattery system. Once separated from the electrolyte, the hydrogen gasmay fill the gas head spaces 90 and 92. As such, the stored hydrogen gascan aid in purging other gases from the multi-chamber storage tank 110,thereby acting as an inert gas blanket for reducing oxidation ofelectrolyte species, which can help to reduce redox flow batterycapacity losses. In this way, utilizing the integrated multi-chamberedstorage tank 110 may forego having separate negative and positiveelectrolyte storage tanks, hydrogen storage tanks, and gas-liquidseparators common to conventional redox flow battery systems, therebysimplifying the system design, reducing the physical footprint of thesystem, and reducing system costs.

FIG. 1 also shows the spill over-hole 96, which creates an opening inthe bulkhead 98 between gas head spaces 90 and 92, and provides a meansof equalizing gas pressure between the two chambers. The spill over hole96 may be positioned a threshold height above the fill height 112. Thespill over hole further enables a capability to self-balance theelectrolytes in each of the positive and negative electrolyte chambersin the event of a battery crossover. In the case of an all iron redoxflow battery system, the same electrolyte (Fe²⁺) is used in bothnegative and positive electrode compartments 20 and 22, so spilling overof electrolyte between the negative and positive electrolyte chambers 50and 52 may reduce overall system efficiency, but the overall electrolytecomposition, battery module performance, and battery module capacity aremaintained. Flange fittings may be utilized for all piping connectionsfor inlets and outlets to and from the multi-chambered storage tank 110to maintain a continuously pressurized state without leaks. Themulti-chambered storage tank 110 can include at least one outlet fromeach of the negative and positive electrolyte chambers, and at least oneinlet to each of the negative and positive electrolyte chambers.Furthermore, one or more outlet connections may be provided from the gashead spaces 90 and 92 for directing hydrogen gas to rebalancing reactors80 and 82.

Although not shown in FIG. 1 , integrated multi-chambered electrolytestorage tank 110 may further include one or more heaters thermallycoupled to each of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52. In alternate examples, only one of the negativeand positive electrolyte chambers may include one or more heaters. Inthe case where only the positive electrolyte chamber 52 includes one ormore heaters, the negative electrolyte may be heated by transferringheat generated at the battery cells of the power module to the negativeelectrolyte. In this way, the battery cells of the power module may heatand facilitate temperature regulation of the negative electrolyte. Theone or more heaters may be actuated by the controller 88 to regulate atemperature of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52 independently or together. For example, inresponse to an electrolyte temperature decreasing below a thresholdtemperature, the controller 88 may increase a power supplied to one ormore heaters so that a heat flux to the electrolyte is increased. Theelectrolyte temperature may be indicated by one or more temperaturesensors mounted at the multi-chambered electrolyte storage tank 110,including sensors 60 and 62. As examples, the one or more heaters mayinclude coil type heaters or other immersion heaters immersed in theelectrolyte fluid, or surface mantle type heaters that transfer heatconductively through the walls of the negative and positive electrolytechambers to heat the fluid therein. Other known types of tank heatersmay be employed without departing from the scope of the presentdisclosure. Furthermore, controller 88 may deactivate one or moreheaters in the negative and positive electrolyte chambers 50, 52 inresponse to a liquid level decreasing below a solids fill thresholdlevel. Said in another way, controller 88 may activate the one or moreheaters in the negative and positive electrolyte chambers 50, 52 only inresponse to a liquid level increasing above the solids fill thresholdlevel. In this way, activating the one or more heaters withoutsufficient liquid in the positive and/or negative electrolyte chamberscan be averted, thereby reducing a risk of overheating or burning outthe heaters.

Further still, one or more inlet connections may be provided to each ofthe negative and positive electrolyte chambers 50, 52 from a fieldhydration system (not shown). In this way, the field hydration systemcan facilitate commissioning of the redox flow battery system, includinginstalling, filling, and hydrating the system, at an end-use location.Furthermore, prior to its commissioning at the end-use location, theredox flow battery system may be dry-assembled at a batterymanufacturing facility different from end-use location without fillingand hydrating the system, before delivering the system to the end-uselocation. In one example, the end-use location may correspond to thelocation where the redox flow battery system 10 is to be installed andutilized for on-site energy storage. Said in another way, it isanticipated that, once installed and hydrated at the end-use location, aposition of the redox flow battery system 10 becomes fixed, and theredox flow battery system 10 is no longer deemed a portable, dry system.Thus, from the perspective of a redox flow battery system end-user, thedry portable redox flow battery system 10 may be delivered on-site,after which the redox flow battery system 10 is installed, hydrated andcommissioned. Prior to hydration the redox flow battery system 10 may bereferred to as a dry, portable system, the redox flow battery system 10being free of or without water and wet electrolyte. Once hydrated, theredox flow battery system 10 may be referred to as a wet non-portablesystem, the redox flow battery system 10 including wet electrolyte.

Further illustrated in FIG. 1 , electrolyte solutions typically storedin the multi-chambered storage tank 110 are pumped via negative andpositive electrolyte pumps 30 and 32 throughout the flow battery system10. Electrolyte stored in negative electrolyte chamber 50 is pumped vianegative electrolyte pump 30 through the negative electrode compartment20 side, and electrolyte stored in positive electrolyte chamber 52 ispumped via positive electrolyte pump 32 through the positive electrodecompartment 22 side of the battery.

Two electrolyte rebalancing reactors 80 and 82, may be connected in-lineor in parallel with the recirculating flow paths of the electrolyte atthe negative and positive sides of the battery cell 18, respectively, inthe redox flow battery system 10. One or more rebalancing reactors maybe connected in-line with the recirculating flow paths of theelectrolyte at the negative and positive sides of the battery, and otherrebalancing reactors may be connected in parallel, for redundancy (e.g.,a rebalancing reactor may be serviced without disrupting battery andrebalancing operations) and for increased rebalancing capacity. In oneexample, the electrolyte rebalancing reactors 80 and 82 may be placed inthe return flow path from the positive and negative electrodecompartments 20 and 22 to the positive and negative electrolyte chambers50 and 52, respectively. Electrolyte rebalancing reactors 80 and 82 mayserve to rebalance electrolyte charge imbalances in the redox flowbattery system occurring due to side reactions, ion crossover, and thelike, as described herein. In one example, electrolyte rebalancingreactors 80 and 82 may include trickle bed reactors, where the hydrogengas and electrolyte are contacted at catalyst surfaces in a packed bedfor carrying out the electrolyte rebalancing reaction. In other examplesthe rebalancing reactors 80 and 82 may include flow-through typereactors that are capable of contacting the hydrogen gas and theelectrolyte liquid and carrying out the rebalancing reactions in theabsence of a packed catalyst bed.

During operation of the redox flow battery system 10, sensors and probesmay monitor and control chemical properties of the electrolyte such aselectrolyte pH, concentration, state of charge, and the like. Forexample, as illustrated in FIG. 1 , sensors 62 and 60 maybe bepositioned to monitor positive electrolyte and negative electrolyteconditions at the positive electrolyte chamber 52 and the negativeelectrolyte chamber 50, respectively. In another example, sensors 62 and60 may each include one or more electrolyte level sensors to indicate alevel of electrolyte in the positive electrolyte chamber 52 and thenegative electrolyte chamber 50, respectively. As another example,sensors 72 and 70, also illustrated in FIG. 1 , may monitor positiveelectrolyte and negative electrolyte conditions at the positiveelectrode compartment 22 and the negative electrode compartment 20,respectively. Sensors may be positioned at other locations throughoutthe redox flow battery system 10 to monitor electrolyte chemicalproperties and other properties.

For example, a sensor may be positioned in an external acid tank (notshown) to monitor acid volume or pH of the external acid tank, whereinacid from the external acid tank is supplied via an external pump (notshown) to the redox flow battery system 10 in order to reduceprecipitate formation in the electrolytes. Additional external tanks andsensors may be installed for supplying other additives to the redox flowbattery system 10. For example, various sensors including, temperature,conductivity, and level sensors of a field hydration system may transmitsignals to the controller 88. Furthermore, controller 88 may sendsignals to actuators such as valves and pumps of the field hydrationsystem during hydration of the redox flow battery system 10. Sensorinformation may be transmitted to a controller 88 which may in turnactuate pumps 30 and 32 to control electrolyte flow through the cell 18,or to perform other control functions, as an example. In this manner,the controller 88 may be responsive to one or a combination of sensorsand probes.

Redox flow battery system 10 may further comprise a source of hydrogengas. In one example the source of hydrogen gas may comprise a separatededicated hydrogen gas storage tank. In the example of FIG. 1 , hydrogengas may be stored in and supplied from the integrated multi-chamberedelectrolyte storage tank 110. Integrated multi-chambered electrolytestorage tank 110 may supply additional hydrogen gas to the positiveelectrolyte chamber 52 and the negative electrolyte chamber 50.Integrated multi-chambered electrolyte storage tank 110 may alternatelysupply additional hydrogen gas to the inlet of electrolyte rebalancingreactors 80 and 82. As an example, a mass flow meter or other flowcontrolling device (which may be controlled by controller 88) mayregulate the flow of the hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110. The integrated multi-chambered electrolytestorage tank 110 may supplement the hydrogen gas generated in redox flowbattery system 10. For example, when gas leaks are detected in redoxflow battery system 10 or when the reduction reaction rate is too low atlow hydrogen partial pressure, hydrogen gas may be supplied from theintegrated multi-chambered electrolyte storage tank 110 in order torebalance the state of charge of the electro-active species in thepositive electrolyte and negative electrolyte. As an example, controller88 may supply hydrogen gas from integrated multi-chambered electrolytestorage tank 110 in response to a measured change in pH or in responseto a measured change in state of charge of an electrolyte or anelectro-active species.

For example, an increase in pH of the negative electrolyte chamber 50,or the negative electrode compartment 20, may indicate that hydrogen isleaking from the redox flow battery system 10 and/or that the reactionrate is too slow with the available hydrogen partial pressure, andcontroller 88, in response to the pH increase, may increase a supply ofhydrogen gas from integrated multi-chambered electrolyte storage tank110 to the redox flow battery system 10. As a further example,controller 88 may supply hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110 in response to a pH change, wherein the pHincreases beyond a first threshold pH or decreases beyond a secondthreshold pH. In the case of an IFB, controller 88 may supply additionalhydrogen to increase the rate of reduction of ferric ions and the rateof production of protons, thereby reducing the pH of the positiveelectrolyte. Furthermore, the negative electrolyte pH may be lowered byhydrogen reduction of ferric ions crossing over from the positiveelectrolyte to the negative electrolyte or by protons, generated at thepositive side, crossing over to the negative electrolyte due to a protonconcentration gradient and electrophoretic forces. In this manner, thepH of the negative electrolyte may be maintained within a stable region,while reducing the risk of precipitation of ferric ions (crossing overfrom the positive electrode compartment) as Fe(OH)₃.

Other control schemes for controlling the supply rate of hydrogen gasfrom integrated multi-chambered electrolyte storage tank 110 responsiveto a change in an electrolyte pH or to a change in an electrolyte stateof charge, detected by other sensors such as an oxygen-reductionpotential (ORP) meter or an optical sensor, may be implemented. Furtherstill, the change in pH or state of charge triggering the action ofcontroller 88 may be based on a rate of change or a change measured overa time period. The time period for the rate of change may bepredetermined or adjusted based on the time constants for the redox flowbattery system 10. For example, the time period may be reduced if therecirculation rate is high, and local changes in concentration (e.g.,due to side reactions or gas leaks) may quickly be measured since thetime constants may be small.

An IFB system, e.g., the flow battery system 10 of FIG. 1 , may includea plurality of battery cells, such as the battery cell 18 of FIG. 1 ,stacked along a common axis and forming a cell stack. An IFB of the IFBsystem may further include a variety of hardware that provide structuralsupport to the IFB and enable an assembly of the hardware and the cellinto a transportable unit. A first example embodiment of an IFB 202 isshown in FIG. 2 from a perspective view 200. A set of reference axis 201is provided, indicating a y-axis, an x-axis, and a z-axis. The IFB 202may have a sandwich structure with a cell stack 204 sandwiched between afirst pressure plate 206 and a second pressure plate 208, the cell stack204 and pressure plates aligned along a longitudinal axis, e.g., thez-axis.

The first pressure plate 206 may be arranged at a first end 210 and thesecond pressure plate 208 may be arranged at a second end 212 of the IFB202. The first and second pressure plates 206, 208 may be end walls ofthe IFB 202 lending rigid support to the IFB 202 as well as compressionalong the z-axis. For example, the first and second pressure plates 206,208 allow the cell stack 204 to be compressed between the pressureplates to seal components as well as electrolyte of the IFB 202 withinan interior 402 of the IFB 202, the interior 402 indicated in anexploded view 400 of the IFB 202 shown in FIG. 4 .

Turning now to FIG. 4 , elements of the IFB 202 are now described alonga direction from the first end 210 towards the second end 212. The cellstack 204 includes a first, or negative end plate 404 positioned insideof the first pressure plate 206 and a first current collector 406arranged between the negative end plate 404 and the first pressure plate206. The first current collector 406 may have a shorter height along they-axis than either of the negative end plate 404 and the first pressureplate 206. The negative end plate 404 may be at least partially inface-sharing contact with an interior surface of the first pressureplate 206. In other words, a surface of the first pressure plate 206facing the interior 402 of the IFB 202 is in contact with a surface ofthe negative end plate 404 facing away from the interior 402 of the IFB202 around the first current collector 406.

A bipolar plate assembly 408 is arranged between the negative end plate404 and a second, positive end plate 410 of the cell stack 204. Thebipolar plate assembly 408 includes a plurality of frame plates 412stacked along the z-axis, the plurality of frame plates 412 providingstructural support to the cell stack 204. Each frame plate of theplurality of frame plates 412 is similarly configured to frame one ormore cells of the first cell stack 312 and includes a bipolar plate 414inserted into each of at least one opening of each frame plate.Furthermore, the bipolar plate 414 is positioned between a negativeelectrode and a positive electrode of the one or more cells (as shown inFIGS. 7-10 ), the electrodes arranged along opposite faces of thebipolar plate 414. The negative electrode is positioned between thebipolar plate 414 and a membrane separator, such as the separator 24 ofFIG. 1 . In this way, each frame plate has a stack of componentsincluding the membrane separator, the negative electrode, the bipolarplate 414, and the positive electrode, and the stack of components isrepeated with each frame plate in the cell stack 204.

As described above, the cell stack 204 extends along the z-axis from thenegative end plate 404 at the first end 210 of the IFB 202 to thepositive end plate 410 at the second end 212 of the IFB 202. A secondcurrent collector 416, similar to the first current collector 406 may bepositioned between the positive end plate 410 and the second pressureplate 208, the second current collector 416 also configured to flowelectrical current. The second current collector 416 may be shorter inheight, along the y-axis, than either the positive end plate 410 or thesecond pressure plate 208. The positive end plate 410 may be at leastpartially in face-sharing contact with the second pressure plate 208. Inother words, a surface of the positive end plate 410 facing away fromthe interior 402 of the IFB 202 is in contact with a surface of thesecond pressure plate 208 facing the interior 402 of the IFB 202, aroundthe second current collector 416.

The negative and positive end plates 404, 410 may provide solid endwalls to the cell stack to seal fluids, such as electrolyte, inside thecell stack 204. The cell stack 204 may be secured to the first andsecond pressure plates 206, 208, by a variety of hardware, including,for example, bolts 418, nuts 420, and other types of fasteners. Thefirst and second pressure plates 206, 208, enable the IFB 202 to becompressed by a desired amount of force, such as 209 kN, as one example.Compression of the IFB 202 is maintained by the fasteners which securecomponents of the IFB together in series along the z-axis.

It will be appreciated that the IFB 202 shown in FIG. 2 is anon-limiting example of an IFB which may be adapted with the elementsdescribed herein. While the IFB 202 shows one cell stack 204 compressedbetween the first pressure plate 206 and the second pressure plate 208,other examples may include systems with differing numbers of cellstacks. As an example, a second embodiment of an IFB 302 is illustratedin FIG. 3 in a perspective view 300. The IFB 302 has a first pressureplate 304 at a first end 306 of the IFB 302 and a second pressure plate308 at a second end 310 of the IFB 302.

A first cell stack 312 and a second cell stack 314 may be disposedbetween the first and second pressure plates 304, 308. Each of the firstcell stack 312 and the second cell stack 314 may be similar to the cellstack 204 of the IFB 202 of FIGS. 2 and 4 . The first cell stack 312 maybe aligned along the z-axis and separated by a sub-stack separator plate316 that blocks exchange of fluid between the cell stacks. In this way,an energy storage capacity of the IFB 302 may be increased relative tothe IFB 202 of FIGS. 2 and 4 without increasing a number of hardwarecomponents, such as pressure plates, bolts, nuts, etc., to assemble theIFB into a transportable unit.

A rear view 500 of the IFB 202 of FIGS. 2 and 4 is shown in FIG. 5 . Inanother example, the IFB shown in FIG. 5 may be the second embodiment ofthe IFB 302 depicted in FIG. 3 . The second pressure plate 208, arrangedat the second end 212 of the IFB 202, as shown in FIGS. 2 and 4 , mayhave a plurality of ports 502 extending through a thickness of thesecond pressure plate 208 where the thickness is defined along thez-axis. In one example, the second end 212 of the IFB 202 may be apositive end of the IFB 202. A first port 504 of the plurality of ports502 may be a negative inlet, flowing negative electrolyte into the IFB202. A second port 506 of the plurality of ports 502 may be a positiveinlet, flowing positive electrolyte into the IFB 202. A third port 508of the plurality of ports 502 may be a negative outlet port, flowingnegative electrolyte out of the IFB 202, and a fourth port 510 of theplurality of ports 502 may be a positive outlet port, flowing positiveelectrolyte out of the IFB 202. As such all electrolyte entering the IFB202 enters through the second pressure plate 208 and all electrolyteleaving the IFB 202 exits through the second pressure plate 208.

Both the first port 504 and the second port 506 may be positioned at abottom end 512, with respect to the y-axis, of the IFB 202, and may bealigned with one another along the x-axis. The third port 508 and thefourth port 510 may be positioned at a top end 514, with respect to they-axis, of the IFB 202, also aligned with one another along the x-axis.Thus, negative electrolyte follows a negative flow path into the IFB 202at the first port 504 along a first direction from the second pressureplate 208 to the first pressure plate 206, as shown in FIG. 6 in a sideview 600 of the IFB 202 and indicated by arrow 602. The negative flowpath may turn to the left (with respect to the rear view 500 shown inFIG. 5 ) and upwards (along the y-axis) as indicated by arrows 604 atmore than one location along the z-axis between the second pressureplate 208 and the first pressure plate 206. The flow path may makeanother perpendicular turn to flow along a second direction from thefirst pressure plate 206 to the second pressure plate 208, as indicatedby arrow 606, to emerge from the third port 508.

Similarly, positive electrolyte follows a positive flow path into theIFB 202 at the second port 506 along the first direction, indicated byarrow 602 in FIG. 6 , the positive flow path turning to the right (withrespect to the rear view 500 shown in FIG. 5 ) and upwards (along they-axis) at locations along the z-axis between the second pressure plate208 and the first pressure plate 206. The flow path turns again,perpendicularly, to flow along the second direction, as indicated byarrow 606 in FIG. 6 , from the first pressure plate 206 to the secondpressure plate 208 to emerge from the fourth port 510.

It will be appreciated that the IFB 202 shown in FIGS. 2 and 4-6 is anon-limiting example and other examples may include variations inelectrolyte flow paths without departing from the scope of the presentdisclosure. For example, the electrolyte flow paths may instead enterthe IFB at an upper region of the IFB and exit the IFB at a lower regionof the IFB, or the electrolyte flow paths may enter and exit at amid-region between the upper and lower regions. Furthermore, a negativeelectrolyte or a positive electrolyte may flow in and out of the IFBalong a same side of the IFB rather than opposite sides.

A performance of an IFB may be affected by a flow rate of electrolytewithin a battery cell of the IFB as well as by contact between theelectrolyte and surfaces of components within the battery cell. Theelectrolyte flow rate may be moderated by a permeability of electrodesof the battery cell and contact between the electrolyte and reactivecomponents, such as the membrane separator, may be regulated by ageometry of battery cell components. For example, one or more of thebattery cell components may be adapted with channels to guideelectrolyte flow, thereby increasing flow rates through the battery celland promoting contact between the electrolyte and target regions of thebattery cell.

In one example, a felt may be used to form the positive electrode. Thefelt may be a material such as carbon or graphite, or some othercost-effective, conductive material, which, when compressed via pressureexerted against the felt by the negative electrode, may provide regionsof high and low permeability, thereby controlling electrolyte flowtherethrough. Flow across the negative electrode may also affect aperformance of the IFB. When formed from felt, the negative electrodebecome clogged when iron is plated onto the negative electrode during acharging cycle of the IFB. As an alternative, the negative electrode maybe configured as a mesh, thus increasing a surface area of the negativeelectrode and allowing electrolyte to readily flow along surfaces of thenegative electrode.

The felt of the positive electrode may be porous, deformable andcompressible. By implementing the felt positive electrode in the IFBrather than, for example, a GDL paper, a compression of the IFB betweena set of pressure plates, e.g., the first and second pressure plates206, 208 of FIGS. 2 and 4 , may be reduced. The reduced compressionallows use of less costly, lower load-bearing components in the IFB.Furthermore, incorporating the more permeable and compressible positiveelectrode in the IFB, e.g., compared to other components in an electrodeassembly of the IFB, may preclude use of a costly net-shape moldedbipolar plate, enabling adjustment of electrolyte flow by modifyingphysical properties of the positive electrode rather than imprintingIDFF channels into the bipolar plate.

Within the battery cell, the negative electrode and the positiveelectrode may be separated by the membrane separator. The membraneseparator may be thin and flexible which allows a compressive forceexerted by the negative electrode to be transmitted through the membraneseparator and imposed on the positive electrode. Regions where thenegative electrode mesh contacts the membrane separator may representregions of increased compression of the positive electrode, wherepressure exerted on the positive electrode felt is supported by thebipolar plate arranged on an opposite side of the positive electroderelative to the membrane separator. A positioning of the bipolar plateresists the applied pressure from the negative electrode, resulting incompression of the positive electrode felt and altering a permeabilityof the positive electrode at the regions of compression. Details ofcompression of the positive electrode and positioning of othercomponents of an electrode assembly for the IFB are described furtherbelow with reference to FIGS. 7-14 .

Turning now to FIG. 7 , an exploded view 700 of a first example of anelectrode assembly 702 for an IFB is shown. The electrode assembly 702has a central axis 701 extending longitudinally through a central pointof each layer of the electrode assembly 702, the central axis 701 beingparallel with the z-axis. Components of the electrode assembly 702 arestacked along the central axis 701 and may include, along a directionfrom a top to a bottom of the stack, a bipolar plate assembly 704, amesh 706, and a membrane separator assembly 708. In the IFB, theelectrode assembly 702 may be repeated numerous times sequentially sothat each electrode assembly is adjacent to at least one other electrodeassembly 702, where all the electrode assemblies are stacked within acell stack and aligned along the z-axis, as shown in FIGS. 2-6 .Electrolyte may flow through each electrode assembly, following, forexample, flow paths shown in FIG. 6 .

The bipolar plate assembly 704 includes a bipolar frame plate 710framing bipolar plates 712 arranged in openings of the bipolar frameplate 710. The bipolar plates 712 may be, as an example, impermeablecompressed graphite or carbon sheets providing conductivity whileinhibiting exchange of electrolyte between negative electrodecompartments of the electrode assembly 702. The bipolar frame plate 710may provide structural support to the bipolar plates 712 and enablecoupling of the bipolar plate assembly 704 to other components of thecell stack of the IFB. For example, the bipolar frame plate 710 and amembrane frame plate 732 of the membrane separator assembly 708 may eachhave apertures 715 to accommodate fasteners, such as the bolts 418 ofFIG. 4 . The apertures 715 may be disposed in tabs 717 which maystructurally reinforce the fasteners, enabling greater force dispersionin a cell stack of the IFB. In other examples, however, other platecontours may be used.

The electrode assembly 702 also includes a negative electrolyte inlet716, a negative electrolyte outlet 718, a positive electrolyte inlet720, and a positive electrolyte outlet 722, which are at least partiallydisposed in the bipolar plate assembly 704 and partially disposed in themembrane separator assembly 708. It will be understood that theelectrolyte inlets and outlets are formed via the mating between thebipolar plate assembly 704 and the membrane separator assembly 708.

A first surface 724 of the bipolar plate assembly 704, e.g., a face ofthe bipolar plate assembly 704 facing upwards along the z-axis, mayinclude positive shunt channels 726 extending from the positiveelectrolyte inlet 720 and positive electrolyte outlet 722. The positiveshunt channels 726 may be serpentine in shape to allow a length of thepositive shunt channels 726 to be increased to reduce shunt current.Consequently, the battery system may be operated more efficiently withregard to energy power output and in some cases storage capacity. Itwill be appreciated that the cross-sectional area of the shunt channelsmay also be decreased to reduce shunt current, in certain examples.Furthermore, other suitable electrolyte flow paths in the positive shuntchannels 726 have been envisioned.

In addition, the first surface 724 of the bipolar may also includepositive inlet and outlet distribution channels 728. The positive inletand outlet distribution channels 728 enable positive electrolyte to bedistributed and captured from an active area 730 of the electrodeassembly 702. Thus, the positive inlet and outlet distribution channels728 may be in fluidic communication with the positive shunt channels726.

An outer geometry of the bipolar frame plate 710 may match an outergeometry of the membrane frame plate 732 of the membrane separatorassembly 708. The membrane frame plate 732, similar to the bipolar frameplate 710 and the bipolar plates 712, may frame membranes 734 arrangedin openings of the membrane frame plate 732. The membranes 734 may besimilar in size and shape to the bipolar plates 712 and may be alignedwith the bipolar plates 712 along the z-axis. Alternatively, themembranes 734 may not be separate panels but instead one continuousmembrane panel extending entirely across openings in the membrane frameplate 732. The membranes 734 may be non-limiting examples of theseparator 24 of FIG. 1 , configured to enable selective transport ofions to maintain a charge balance between a negative and a positiveelectrolyte. The membranes 734 may be thin and flexible and sealinglyengage with the membrane frame plate 732 so that electrolyte does notflow between the membrane frame plate 732 and edges of the membranes734.

Electrolyte flow channels may be formed at an interface of the bipolarplate assembly 704 and the membrane separator assembly 708. Toelaborate, negative shunt channels 736 may extend from the negativeelectrolyte inlet 716 and the negative electrolyte outlet 718 ofelectrode assembly 702, when assembled, along a first surface 738 of themembrane separator assembly 708, e.g., a face of the membrane separatorassembly 708 facing upwards along the z-axis. The negative shuntchannels 736 may also extend along a second surface 740 of the bipolarplate assembly 704, e.g., a face of bipolar plate assembly 704 facingdownwards along the z-axis. The negative shunt channels 736 may beserpentine in shape, similar to the positive shunt channels 726. Themembrane separator assembly 708 may also include negative distributionchannels 742 to enable negative electrolyte to be distributed andcaptured from the active area 730 of the electrode assembly 702. Thus,the negative distribution channels 742 may be in fluidic communicationwith the negative shunt channels 736.

It will be appreciated that the general flow path the electrolyte (e.g.,positive or negative electrolyte) in the electrode assembly 702 proceedsas follows: (i) electrolyte initially flows through an electrolyte inletinto a corresponding shunt channel; (ii) electrolyte then flows from theshunt channel into the inlet distribution channels; (iii) electrolytethen flows from the inlet distribution channels into themembrane/bipolar plate interface; (iv) electrolyte then flows from themembrane/bipolar plate interface into the outlet distribution channels;(v) electrolyte then flows from the outlet distribution channels toassociated shunt channels; and (vi) subsequently the electrolyte flowsfrom the shunt channels into a respective electrolyte outlet.

The mesh 706, arranged between the bipolar plate assembly 704 and themembrane separator assembly 708, may be divided into panels 744 withdimensions matching those of the bipolar plates 712 and the membranes734. More specifically, the panels 744 of the mesh 706 may be configuredto be sandwiched, e.g., stacked, between each of the bipolar plates 712and each of the membranes 734 aligned along the z-axis. The bipolarplates 712 may therefore be spaced away from the membranes 734 at leastby a thickness of the mesh 706, the thickness defined along the centralaxis 701.

The mesh 706 may include ribs and cross-bracing extending between andstructurally reinforcing the ribs. In one example, the mesh 706 may beconstructed out of a suitable polymer (e.g., polypropylene), allowingfor structural reinforcement of the cell stack without chemicallyinterfering with the electrolyte. In another example, the mesh 706 maybe configured as a negative electrode in addition to providingstructural reinforcement to the electrode assembly 702. As such, themesh 706 may be coated with a hydrophilic or conductive layer. Forexample, the coating may include a carbon ink, a metal oxide or ahydrophilic polymer, promoting plating and deplating of iron alongsurfaces of the mesh 706. The mesh 706 may be in direct contact withnegative electrolyte flowing between the second surface 740 of thebipolar plate assembly 704 and the first surface of the membraneseparator assembly 708, defining negative electrolyte flow pathstherebetween, as described further below with reference to FIGS. 10-13 .

The membrane frame plate 732 and/or the bipolar frame plate 710 may beconstructed out of a suitable polymer such as chlorinated polyvinylchloride (CPVC) and the like. The membranes 734 may be constructed outof a coated Nafion™, in one use-case example. However, other suitablemembrane materials are envisioned. When assembled, the membraneseparator assembly 708 and the bipolar plate assembly 704 may beadhesively bonded together. Adhesive bonding may also be used to adherethe membranes 734 to the membrane frame plate 732 and/or the bipolarplates 712 to the bipolar frame plate 710. However, other suitableattachment techniques such as heat welding have also been contemplatedfor attaching these components.

To form the cell stack of the IFB, a positive electrode is also includedin the electrode assembly 702. A partially exploded view 800 of theelectrode assembly 702 is shown in FIG. 8 and the fully assembledelectrode assembly 702 is depicted in FIG. 9 . The bipolar plateassembly 704 is shown coupled to the membrane separator assembly 708with the mesh 706 arranged in between in FIGS. 8 and 9 , forming asingle, continuous unit. A felt 802 may be coupled to the first surface724 of the bipolar plate assembly 704. More specifically, the felt 802may be divided into panels 804 similar in size and shape to the bipolarplates 712 of the bipolar plate assembly 704. Each of the panels 804 ofthe felt 802 may be face-sharing contact with the first surface 724 ofeach of the bipolar plates 712.

When assembled, the felt 802 may be maintained within the openings ofthe bipolar frame plate 710 by adhesive, heat welding etc. The felt 802may form a positive electrode of the electrode assembly 702, in directcontact with the positive electrolyte of the IFB and may have specificphysical properties that enhance a performance of the IFB. For example,the felt 802 may have a thickness, defined along the z-axis, of between2 and 6.5 mm to provide a desired permeability of electrolyte throughthe felt 802. However, the thickness of the felt 802 may vary dependingon the porosity of the felt and/or a viscosity of the positiveelectrolyte. As another example, the felt may have a compressibilitybetween 5 to 60%. The compressibility may affect an amount ofcompression imposed on the entire IFB to achieve a desired masstransport across the electrode assembly 702, thus controlling anefficiency of the IFB.

When the battery cells of the IFB are stacked and compressed togetherbetween a set of pressure plates, shallow compartments may be formedalong each surface of the bipolar plates 712, each shallow compartmentforming either a negative electrode compartment, along the secondsurface 740 of each of the bipolar plates 712 or a positive electrodecompartment, along the first surface 724 of each of the bipolar plates712. The negative electrode compartment may be filled with the negativeelectrolyte and the positive electrode compartment filled with thepositive electrolyte. Each compartment is sealed by contact betweensurfaces of the bipolar frame plate 710 and surfaces of the membraneframe plate 732 as the frames are pressed together along the z-axis.

One or more of the electrode assembly 702 may be included in the cellstack of the IFB. The electrode assemblies 702 may be stacked along thez-axis so that the positive electrode compartments and negativeelectrode compartments are ordered in an alternating pattern along thez-axis. Stacking of the electrode assemblies is shown in greater detailin FIG. 10 in a section of a cross-sectional view 1000 of an electrodeassembly. For example, the section of the cross-sectional view 1000 maybe taken along a line similar to line A-A′ (as shown in FIG. 9 ) of acell stack of an IFB.

A cell stack 1050 is illustrated in FIG. 10 , including a firstelectrode assembly 1003 stacked over a second electrode assembly 1005along the z-axis. Each of the first electrode assembly 1003 and thesecond electrode assembly 1005 may be similarly configured to theelectrode assembly 702 of FIGS. 7-9 . The first electrode assembly 1003has a first membrane separator assembly 1007 with a first membrane frameplate 1052, a first membrane 1002, and a first negative electrode 1004,as well as a first bipolar plate assembly 1009 with a first bipolarframe plate 1054, a first bipolar plate 1006, and a first positiveelectrode 1008. Each component of the first electrode assembly 1003 maybe in contact with a component directly above and a component directlybelow. Similarly, the second electrode assembly 1005 has a secondmembrane separator assembly (not shown in FIG. 10 ) with a secondmembrane frame plate 1056, a second bipolar plate assembly 1011 with asecond bipolar frame plate 1058, a second negative electrode 1012, asecond bipolar plate 1014, and a second positive electrode 1010positioned immediately below the first membrane 1002. Each component ofthe second electrode assembly 1005 is in contact with a componentdirectly above and a component directly below.

As described above, first negative electrode 1004 and the secondnegative electrode 1012 may be formed from a mesh. As such, eachnegative electrode has sections of material spaced uniformly apart alongthe x-axis and the y-axis by openings in the mesh. For example, thefirst negative electrode 1004 has a plurality of ribs 1016 arrangeddirectly below and in contact with the first bipolar plate 1006 anddirectly above and in contact with the first membrane 1002.Cross-bracing 1015 may extend between the plurality of ribs 1016 toconnect the plurality of ribs 1016 to one another. It will beappreciated that the cross-bracing 1015 includes spaces between thecross-bracing 1015 along the y-axis, where the spaces may be similar tothe spaces between each of the plurality of ribs 1016. Thus, thenegative electrodes may include uniformly spaced apart openingsextending entirely through a thickness of the negative electrodes.

The first bipolar plate 1006 (as well as the second bipolar plate 1014)may be thicker and more rigid than the first membrane 1002. Compressiveforces imposed on the IFB are thereby readily transmitted through thefirst membrane 1002. For example, when the cell stack 1050 is compressedbetween a set of pressure plates, a first applied pressure may beexerted on the second positive electrode 1010, the first membrane 1002,and the first negative electrode 1004 in a downwards direction, withrespect to the z-axis, by the first bipolar plate 1006, as indicated byarrow 1018. An opposing, second applied pressure may be exerted on thesecond positive electrode 1010, the first membrane 1002, and the firstnegative electrode 1004 in an upwards direction, with respect to thez-axis, by the second bipolar plate 1014, as indicated by arrow 1020.Thus, the second positive electrode 1010, the first membrane 1002, andthe first negative electrode 1004 are compressed between the firstbipolar plate 1006 and the second bipolar plate 1014.

Of the positive electrodes, the membranes, and the negative electrodes,only the positive electrodes are formed from a substantiallycompressible material. As described above, a felt of the positiveelectrodes may be configured to compress by a predetermined amount,based on an anticipated amount of applied compression. A mesh of thenegative electrodes may be formed from a more rigid material, such asplastic, with low compressibility. As shown in FIG. 10 , a planar,bottom surface 1022 of the second positive electrode 1010 may be inface-sharing contact with a planar, top surface 1024 of the secondbipolar plate 1014. Pressure exerted on the second positive electrode1010 by the second bipolar plate 1014, which is indicated by arrow 1020,is therefore uniformly applied across a surface area of the bottomsurface 1022 of the second positive electrode 1010.

At a planar, top surface 1026 of the second positive electrode 1010, thesecond positive electrode 1010 is spaced away from the first bipolarplate 1006 by the first negative electrode 1004. A compressive forceexerted by the first bipolar plate 1006, which is indicated by arrow1018, is transmitted to the top surface 1026 of the second positiveelectrode 1010 through the plurality of ribs 1016 of the first negativeelectrode 1004 and the first membrane 1002. Thus compression at the topsurface 1026 of the second positive electrode 1010 is localized andfocused at intersections of the plurality the of ribs 1016 of the firstnegative electrode 1004 and the second positive electrode 1010.

For example, compression of the second positive electrode 1010 may occurat a first set of zones 1028, below and aligned with each of theplurality of ribs 1016 of the first negative electrode 1004 along thez-axis. The plurality of ribs 1016 may exert a force on the top surface1026 of the second positive electrode 1010 as indicated by arrow 1018.At the first set of zones 1028, a thickness 1030 of the second positiveelectrode 1010 may be reduced compared to a second set of zones 1032 ofthe second positive electrode 1010, the second set of zones 1032arranged in between the first set of zones 1028. The second set of zones1032 may be aligned, along the z-axis, with the cross-bracing 1015 ofthe first negative electrode 1004 as well as with openings in the firstnegative electrode 1004, e.g., aligned with spaces between thecross-bracing 1015 along the y-axis. The second set of zones 1032 mayrepresent portions of the second positive electrode 1010 whichexperience some compression due to transmission along the x-y plane ofcompression exerted by the plurality of ribs 1016 of the first negativeelectrode 1004 but less than an amount of compression imposed at thefirst set of zones 1028.

The first set of zones 1028, hereafter high compression zones 1028, maybe regions of low permeability due to the higher compression of the feltof the second positive electrode 1010. Compression of the felt maydecrease a porosity of the felt, thereby impeding electrolyte flowtherethrough. The second set of zones 1032, hereafter low compressionzones 1032, may have higher permeability than the high compression zones1028. Each positive electrode of the cell stack 1050 may therefore havealternating regions of high permeability and low permeability across thex-y plane, along alternating along both the x-axis and the y-axis. Theregions of high permeability correspond to (e.g., align with along thez-axis) openings in the negative electrode mesh where the spaces may befilled with electrolyte. The regions of high permeability may have amaximum permeability based on a porosity of the uncompressed felt.

By forming regions of high permeability and regions of low permeabilityin the positive electrode, flow channels are created through thepositive electrode, along the high permeability regions, whereelectrolyte flow is high relative to the regions of low permeability.Flow through the electrode assembly, and the cell stack, may be guidedby forming high and low permeability regions in the positive electrodebased on a geometry of the negative electrode. For example, if longerflow channels through the positive electrode are desired, a mesh withlonger spaces between each rib of the plurality of ribs may be used. Asanother example, a mesh with smaller openings may be used if short andnarrow flow channels are desired. Furthermore, an overall surface arearatio of low permeability to high permeability regions may be adjustedbased on a width or depth (along the x-axis and y-axis, respectively) ofthe plurality of ribs and a relative width and depth of the openingsbetween the plurality of ribs.

In the low permeability zones of the positive electrode, flow ofelectrolyte may be reduced by, for example, 70-90%. A small amount ofelectrolyte may be able to penetrate the compressed regions of thepositive electrode. The compression of the low permeability zones,resulting in regions of decreased thickness of the positive electrode,forces the electrolyte to flow closer to the membrane than in the highpermeability zones. The proximity of the electrolyte to the membrane inthe low permeability zones promotes greater contact between theelectrolyte and the membrane, and therefore higher exchange of ionsacross the membrane, increasing an efficiency of the IFB towardsmaintaining charge balance and enhancing a performance of the IFB. Byforming flow channels through the positive electrode via selectivelycompressing regions of the positive electrode, fabrication of IDFFchannels on the bipolar plate may be superfluous, allowing both thepositive electrode and bipolar plate to be formed from low costmaterials with less processing.

A method 1100 for operating a redox flow battery is shown in FIG. 11 .The redox flow battery may be an IFB adapted with at least one cellstack aligned along a longitudinal axis of the IFB. The at least onecell stack includes a plurality of cells and each cell of the pluralityof cells may be formed of one or more electrode assemblies, such as theelectrode assembly 702 of FIG. 7 . A positive electrode of the one ormore electrode assemblies may be formed from felt while a negativeelectrode of the one or more electrode assemblies may be formed frommesh and the positive electrode may be separated from the negativeelectrode by a membrane separator. The felt of the positive electrode isconfigured to be more deformable and more compressible than the negativeelectrode, a bipolar plate of the one or more electrode assemblies, aswell as pressure plates arranged at terminal ends of the IFB. Thepressure plates may compress the at least one cell stack, inducing thenegative electrode to apply a compressive force on the positiveelectrode which may be resisted by an arrangement of the bipolar plateon an opposite side of the positive electrode from the negativeelectrode.

At 1102, the method includes flowing electrolyte through the at leastone cell stack, where electrolyte generally refers to a positiveelectrolyte and a negative electrolyte. Flowing electrolyte through theat least one cell stack may include flowing the positive electrolytethrough high permeability zones, e.g., zones with higher permeabilitythan compressed regions, of the positive electrode at 1104. The highpermeability zones may correspond to, along the longitudinal axis,spaces or openings in the mesh of the negative electrode. The highpermeability zones may be uncompressed regions of the positive electrodewhere flow of the positive electrolyte through the felt is highest.

Flowing electrolyte through the at least one cell stack also includesflowing positive electrolyte through low permeability zones of the feltof the positive electrode at 1106, e.g., zones of lower permeabilitythan the high permeability zones. The low permeability zones maycorrespond to, along the longitudinal axis, alignment with ribs of themesh of the negative electrode. Edges of the ribs of the mesh contactthe membrane separator and press against the membrane separator with anamount of pressure that is transmitted through the membrane separator toa surface of the positive electrode in contact with the membraneseparator. The pressure exerted on the positive electrode by thenegative electrode compresses the felt, e.g., mechanically decreases athickness of the positive electrode relative to when the positiveelectrode is not compressed, of the positive electrode to form the lowpermeability zones. Although the permeability of the low permeabilityzones is reduced compared to the high permeability zones, a small amountof electrolyte may penetrate the low permeability zones. The reducedthickness of the felt at the low permeability zones forces the smallamount of electrolyte to flow adjacent to the membrane separator,thereby enabling ion exchange across the membrane separator between thepositive and the negative electrolytes.

Flowing electrolyte through the at least one cell stack may furtherinclude flowing negative electrolyte along the negative electrode 1108.The negative electrolyte may be guided by the ribs of the mesh of thenegative electrode to flow through the spaces or openings of the meshand contact the membrane separator. At 1110, flowing electrolyte throughthe at least one cell stack may also include exchanging ions across themembrane separator to maintain charge balance between the positive andthe negative electrode. For example, anions such as Cl⁻ or OH⁻ may betransported across the membrane separator when the membrane separator isformed from an anion exchange membrane. Alternatively, cations such asK⁺ or H⁺ may be transported across the membrane separator when themembrane separator is formed from a cation exchange membrane. In anotherexample, the membrane separator may be a microporous substrate enablingexchange of both anions and cations depending on an ionic gradientacross the membrane separator.

In this way, a low cost electrode assembly may be included in an IFBsystem, the electrode assembly incorporating a compressible positiveelectrode. The positive electrode may be formed from a porous andpermeable material such as felt and a permeability of the felt may beadjusted by selectively compressing regions of the positive electrode.By coupling the felt positive electrode with a negative electrode formedfrom a mesh with lower compressibility than the felt, a geometry of themesh determines a pattern of alternating zones of low permeability andhigh permeability along the positive electrode. The low permeabilityzones are formed by pressure exerted on the positive electrode (andtransmitted through a membrane separator) by ribs of the negativeelectrode. The high permeability zones of the positive electrode arealigned with spaces or openings in the mesh of the negative electrode,thus providing regions of the felt with little or no compression. Thelow compression, high permeability zones of the positive electrode allowgreater penetration and flow of electrolyte, thus providing flowchannels that guide electrolyte flow through the electrode assembly. Adrop in pressure along the positive electrode is reduced and areactivity at the positive electrode is maintained high.

The technical effect of implementing a compressible positive electrodein an IFB system is that a performance of the IFB system is enhancedwhile maintaining low overall system costs.

The disclosure also provides support for a redox flow battery,comprising: a cell stack compressed between terminal structures definingends of the redox flow battery, the cell stack formed of a plurality ofcells, and wherein each cell of the plurality of cells includes: adeformable positive electrode having a permeability based on amechanical compression of the positive electrode and in face-sharingcontact with a first face of a membrane separator, a negative electrodeconfigured to be less compressible than the positive electrode andarranged at a second face of the membrane separator opposite from thepositive electrode, and an electrolyte in contact with the positiveelectrode. In a first example of the system, the positive electrode isformed from a porous and electrically conductive felt and wherein athickness and a permeability of the felt is decreased when the felt iscompressed, the thickness defined along a longitudinal axis of the redoxflow battery. In a second example of the system, optionally includingthe first example, a first planar surface of the positive electrode isin face-sharing contact with the membrane separator and a second planarsurface of the positive electrode, opposite of the first planar surface,is in face-sharing contact with a first bipolar plate. In a thirdexample of the system, optionally including the first and secondexamples, the negative electrode is formed from a mesh with ribs thatdirectly contact the membrane separator and wherein the ribs are spacedevenly apart along a plane perpendicular to the longitudinal axis andeach rib extends along the longitudinal axis between the membraneseparator and a second bipolar plate. In a fourth example of the system,optionally including the first through third examples, the negativeelectrode exerts a pressure on the positive electrode through themembrane separator in a first direction parallel with the longitudinalaxis and wherein an amount of the pressure corresponds to an amount ofcompression of the redox flow battery provided by the terminalstructures. In a fifth example of the system, optionally including thefirst through fourth examples, the positive electrode is compressed in afirst set of zones aligned with the ribs of the mesh of the negativeelectrode along the longitudinal axis and the positive electrode is lesscompressed in a second set of zones aligned with spaces between the ribsof the mesh of the negative electrode along the longitudinal axis andwherein the first set of zones alternate with the second set of zonesalong a plane of the positive electrode, the plane perpendicular to thelongitudinal axis. In a sixth example of the system, optionallyincluding the first through fifth examples, the first bipolar plateexerts a pressure on the second planar surface of the positive electrodein a second direction opposite from the pressure exerted by the negativeelectrode and wherein the pressure exerted by the first bipolar plate isuniform across the second planar surface of the positive electrode. In aseventh example of the system, optionally including the first throughsixth examples, the first set of zones of the positive electrode areregions of low electrolyte permeability and wherein the second set ofzones of the positive electrode are regions with high electrolytepermeability. In an eighth example of the system, optionally includingthe first through seventh examples, the second set of zones of thepositive electrode are flow channels enabling higher electrolyte flowthrough the positive electrode than the first set of zones, the flowchannels configured to guide and distribute electrolyte flow through apositive electrode compartment of each cell of the plurality of cells.In a ninth example of the system, optionally including the first througheighth examples, the positive electrode is configured to be morecompressible than the negative electrode, the first bipolar plate, andthe terminal structures of the redox flow battery and more permeablethan the membrane separator and the first bipolar plate.

The disclosure also provides support for an electrode assembly for aredox flow battery, comprising: a bipolar plate, a positive electrodewith a first permeability, in face-sharing contact with the bipolarplate at a first face of the positive electrode and having regions of asecond, reduced permeability, a membrane separator in face-sharingcontact with a second, opposite face of the positive electrode, and anegative electrode in contact with the membrane separator, opposite ofthe positive electrode, and wherein the negative electrode is configuredto form the regions of the second, reduced permeability in the positiveelectrode by compressing the positive electrode in a direction normal toa surface of the positive electrode. In a first example of the system,the positive electrode is formed of a sheet of a compressible, porousmaterial and regions with the first permeability alternate with theregions of the second, reduced permeability along a plane of thepositive electrode. In a second example of the system, optionallyincluding the first example, the system further comprises: a meshforming the negative electrode, the mesh having a plurality of ribsconnected by cross-bracing, the plurality of ribs arranged in a planeparallel with the plane of the positive electrode and the cross-bracingarranged perpendicular to the plurality of ribs, also in the planeparallel with the plane of the positive electrode, and wherein thenegative electrode is less compressible than the positive electrode. Ina third example of the system, optionally including the first and secondexamples, a compressive force exerted by the negative electrode on thepositive electrode is transmitted through the membrane separator andresisted by the bipolar plate and wherein the compressive force isapplied only in regions where the plurality of ribs of the negativeelectrode contacts the membrane separator. In a fourth example of thesystem, optionally including the first through third examples, theregions of the positive electrode aligning, along a direction normal tothe surface of the positive electrode, with the regions where theplurality of ribs of the negative electrode contacts the membraneseparator are the regions of the second, reduced permeability in thepositive electrode. In a fifth example of the system, optionallyincluding the first through fourth examples, electrolyte flow in theregions of the first permeability is greater than electrolyte flow inthe regions of the second, reduced permeability and wherein electrolyteflowing through the regions of the second, reduced permeability flowsthrough zones with a decreased thickness of the positive electroderelative to when the regions of the second, reduced permeability are notcompressed.

The disclosure also provides support for a method for operating a redoxflow battery, comprising: flowing a positive electrolyte through acompressible positive electrode having a first set of zones with a firstpermeability and a second set of zones with a second, lowerpermeability. In a first example of the method, the method furthercomprises: flowing a negative electrolyte across a negative electrode,wherein the negative electrode is formed of a mesh configured to exert acompressive force on the positive electrode. In a second example of themethod, optionally including the first example, flowing the positiveelectrolyte through the positive electrode includes flowing morepositive electrolyte through the first set of zones than the second setof zones. In a third example of the method, optionally including thefirst and second examples, flowing the positive electrolyte through thesecond set of zones includes flowing the positive electrolyte adjacentto a membrane separator positioned between the positive electrode andthe negative electrode and enabling ion exchange between the positiveelectrolyte and a negative electrolyte across the membrane separator.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A redox flow battery, comprising: a cell stack comprising a pluralityof cells, each cell of the plurality of cells comprising: a positiveelectrode and a negative electrode, wherein the positive electrodecomprises a first set of zones with a first permeability and a secondset of zones with a second permeability different than the firstpermeability.
 2. The redox flow battery of claim 1, wherein the secondpermeability is less than the first permeability.
 3. The redox flowbattery of claim 1, wherein a compressibility of first set of zones isless than a compressibility of the second set of zones.
 4. The redoxflow battery of claim 1, wherein zones of the first set of zonesalternate with zones of the second set of zones.
 5. The redox flowbattery of claim 1, wherein a size of each of the first set of zones isdifferent than a size of each of the second set of zones.
 6. The redoxflow battery of claim 1, wherein the positive electrode is a positivefelt electrode arranged between a bipolar plate and a membrane.
 7. Theredox flow battery of claim 1, wherein the positive electrode isdeformable, the negative electrode is configured to be less compressiblethan the positive electrode, and wherein the positive electrode is inface-sharing contact with a first face of a membrane separator and thenegative electrode is in face-sharing contact with a second face of themembrane separate, the second face opposite the first face.
 8. The redoxflow battery of claim 1, wherein the negative electrode is formed from amesh with ribs that directly contact a membrane separator inface-sharing contact with the positive electrode and wherein the ribsare spaced evenly apart along a plane perpendicular to a longitudinalaxis parallel to a thickness of the redox flow battery, and each ribextends along the longitudinal axis between the membrane separator and asecond bipolar plate.
 9. The redox flow battery of claim 8, wherein thepositive electrode is more compressed in the first set of zones alignedwith the ribs of the mesh of the negative electrode along thelongitudinal axis and the positive electrode is less compressed in thesecond set of zones aligned with spaces between the ribs of the mesh ofthe negative electrode along the longitudinal axis and wherein the firstset of zones alternate with the second set of zones along a plane of thepositive electrode, the plane perpendicular to the longitudinal axis.10. An assembly for a redox flow battery, comprising: a positiveelectrode comprising higher permeability regions and lower permeabilityregions; a negative electrode configured to compress the lowerpermeability regions of the positive electrode; and a bipolar platearranged between the positive electrode and the negative electrode. 11.The assembly for the redox flow battery of claim 10, wherein thenegative electrode comprises a plurality of ribs that align with thelower permeability regions of the positive electrode.
 12. The assemblyfor the redox flow battery of claim 11, wherein adjacent ribs of theplurality of ribs are separated by spaces of a plurality of spaces, andwherein the spaces align with the higher permeability regions of thepositive electrode.
 13. The assembly for the redox flow battery of claim11, wherein the plurality of spaces is larger than the plurality ofribs.
 14. The assembly for the redox flow battery of claim 11, whereinthe negative electrode is formed by a mesh, and wherein a cross-bracingis connected to the plurality of ribs, each of the cross-bracing and theplurality of ribs arranged in a plane parallel to a plane of thepositive electrode.
 15. The assembly for a redox flow battery of claim10, wherein a compressive force exerted by the negative electrode on thepositive electrode is transmitted through the membrane separator andresisted by the bipolar plate and wherein the compressive force isapplied only in regions where the plurality of ribs of the negativeelectrode contacts the membrane separator.
 16. A system for a redox flowbattery, comprising: a compressible positive electrode having a firstset of zones with a first permeability and a second set of zones with asecond, lower permeability.
 17. The system for the redox flow battery ofclaim 16, further comprising a negative electrode formed of a mesh andconfigured to exert a compressive force on the second set of zones. 18.The system for the redox flow battery of claim 17, wherein a membraneseparates the compressible positive electrode from the negativeelectrode.
 19. The system for the redox flow battery of claim 18,wherein the negative electrode comprises a plurality of ribs connectedto a cross-bracing, wherein the plurality of ribs and the cross-bracingare in face-sharing contact with the membrane, and wherein only theplurality of ribs is in face-sharing contact with a bipolar plate. 20.The system for the redox flow battery of claim 19, wherein the bipolarplate is a first bipolar plate, further comprising a second bipolarplate in contact with the positive electrode.